We propose to use the parvovirus MVM as a means of investigating the events of eukaryotic DNA synthesis as they occur in vivo. The replication of MVM DNA is known to be dependent upon events which occur only in late S phase of the host cell. The extremely small size of the viral genome and consequent dependence on host cell functions make MVM an ideal tool for the study of host mechanisms that are specific to a portion of the cell cycle. Our objective is to determine which of the steps in viral DNA replication are specifically coupled to the host cell cycle. Two specific areas will be studied. First, we wish to determine if viral DNA integration into the host cell DNA serves as a means of coupling host and virus DNA synthesis. We also propose to determine when in the host cell cycle the viral single-stranded DNA is first converted to its double-stranded replicating form. These experiments involve the isolation of various forms of the replicating viral DNA from highly synchronized, infected cells.